1. Field of the Invention
The present invention relates to & magnetic sensor, and more particularly, to a magnetic impedance sensor which is a high-sensitive magnetic sensor.
2. Description of the Related Art
With the recent rapid development of information devices and measuring and control devices, demand of magnetic sensors which are small sizes, low costs, high in sensitivity and high in response have increased more and more. For example, in a hard disc device of an external memory device for a computer, a high performance has been advanced such that an induction type magnetic head of the bulk type has been changed to a thin film magnetic head or a magnetic resistance effect (MR) head. Since in a rotary encoder which is a rotary sensor for use. in a motor, the number of magnetic poles is increased, there has been required a magnetic sensor which is capable of detecting a fine surface magnetic flux with a high sensitivity instead of the conventional magnetic resistance effect (MR) sensor. Also, demand of a high-sensitive magnetic sensor which can be used for a non-destructive investigation or a bill investigation has increased more and more. Similarly, demand of a small-sized and light-weight azimuth sensor for an automobile, an active magnetic shield sensor for a display tube in a high-accuracy color television or a personal computer, and so on have increased more and more.
As the representative magnetic sensors which are now being used, there are an induction type reproduction magnetic head. a magnetic resistance effect (MR) element, a flux gate sensor, a Hall element and so on. Also, in recent years, there have been proposed magnetic sensors with high sensitivity employing the magnetic impedance effect of an amorphous wire (refer to Japanese Patent Laid-open Publication No. Hei 6-176930, Japanese Patent Laid-open Publication No. Hei 7-181239 and Japanese Patent Laid-open Publication No. Hei 7-333305) and the magnetic impedance effect of a magnetic thin film (refer to Japanese Patent Laid-open Publication No. Hei 8-75835, Japanese Applied Magnetic Institute Journal, vol. 20,553-556 (1996)).
The induction type reproduction magnetic head suffers from such problems that a magnetic head per se is large-sized because a coil winding is required, and also that the sensitivity of detection is remarkably deteriorated because a relative speed between the magnetic head and a medium is low when the induction-type magnetic head is miniaturized. For solving the above problems, the magnetic resistance effect (MR) element using a ferromagnetic film is being employed. The MR element is so designed as to detect not a temporal variation in magnetic flux but the magnetic flux per se, to thereby advance the miniaturizing of the magnetic head. However, in the existing MR element, the rate of change in the electric resistance is about 2%, and even in the MR element using the spin valve element, the rate of change in the electric resistance is small to the degree of 6% or less at the maximum. Also, the external magnetic field necessary for obtaining the in resistance of several % is large to the degree of 1600 A/m or more. Therefore, the magnetic resistance sensitivity is low to the degree of 0.001%/(A/m) or less. Also, in recent years, there has been found a giant magnetic resistance effect (GMR) due to an artificial lattice in which the rate of change in the magnetic resistance is several tens %. However, in order to obtain the change in resistance of several tens %, the external magnetic field of several ten thousands A/m is necessary, and therefore the practical use of the magnetic resistance element as a magnetic sensor has not been realized.
The flux gate sensor which is the conventional high-sensitivity magnetic sensor is so designed as to measure the magnetism by using the phenomenon in which the symmetric B-H characteristic of a high permeability magnetic core such as a permalloy is changed according to the external magnetic field, and has the high resolution and the high directivity of .+-.1.degree.. However, the above flux gate sensor suffers from such problems that a large-sized magnetic core that is small in diamagnetic field is required in order to enhance the sensitivity of detection, thereby making it difficult to reduce the dimensions of the entire sensor and also that the power consumption is large.
The magnetic sensor using a hole element is a sensor using a phenomenon in which when a magnetic field is applied perpendicularly to a surface of the sensor into which a current flows, an electric field is developed in a direction perpendicular to both of the current flowing direction and the magnetic field applying direction. to thereby induce an electromotive force in the hole element. The hole element is advantageous from the viewpoint of the costs but has such defects that the sensitivity of the magnetic field detection is low and that the temperature characteristic of the magnetic field sensitivity is low because the mobility of electrons or positive holes is changed by diffusion of lattices within the semiconductor due to thermal vibrations to a change in temperature since the hole element is made of semiconductor such as Si or GaAs.
Japanese Patent Laid-open Publication No. Hei 6-176930, Japanese Patent Laid-open Publication No. Hei 7-181239 and Japanese Patent Laid-open Publication No. Hei 7-333305 have proposed therein magnetic impedance elements by which a great improvement in the magnetic field sensitivity has been realized. The magnetic impedance element is a magnetic impedance element that has a basic principle in which only a voltage caused when a circumferential magnetic flux changes as a time elapses, using the skin effect that is produced when a current which rapidly varies as a time elapses is supplied to a magnetic line is detected as a change caused by the externally applied magnetic field. An amorphous wire (a wire which has been tension-annealed after having been drawn) which is made of FeCoSiB or the like and about 30 .mu.m in the diameter of exciting distortion is employed as a magnetic line. Even in a wire having a fine dimension of about 1 m in length, when a high-frequency current of about 1 MHz is supplied to the wire, the amplitude of a voltage across the wire changes with the high sensitivity of about 0.1%/(A/m) which is more than 100 times as large as that of the MR element.
Incidentally, as the magnetic sensor, there has been demanded a high-sensitive magnetic sensor which is small size, low costs and excellent in the linearity and the temperature characteristic of an output to the detected magnetic field (external magnetic field) The magnetic sensor using the magnetic impedance effect of the amorphous wire exhibits the magnetic field detection characteristic of a high sensitivity.
Also, Japanese Patent Laid-open Publication No. Hei 6-176930 and Japanese Patent Laid-open Publication No. Hei 6-347489 disclose that the application of a bias magnetic field allows the linearity of the dependency of the applied magnetic field of the impedance change to be improved, and that a negative feedback coil is wound on the amorphous wire, and a voltage between both ends of the amorphous wire is amplified and a negative feedback current is allowed to flow into a negative feedback coil, thereby being capable of providing a magnetic sensor which is excellent in linearity.
Incidentally, Japanese Patent Laid-open Publication No.Hei 6-176930 fails to teach a drive circuit including an oscillating circuit and a detecting circuit, and Japanese Patent Laid-open Publication No. Hei 6-347489 has proposed the combination of a multi-vibrator using a pair of switching transistors with a low-pass filter as an oscillating circuit. Also, Japan Applied Magnetic Society, vol.21. 793-796 (1997) discloses that in order to reduce power consumption, a high-frequency current is applied to amorphous wires by a pulse drive using a C-MOS multi-vibrator, and the respective amorphous wires are detected by diodes to differentially amplify the detected results by the operation amplifier. However, in the circuit of this type, the amount of a magnetic change in the magnetic impedance is not extracted at the maximum, resulting in a problem in view of the temperature characteristic.
A technique using two amorphous wires is disclosed in Japanese Patent Laid-open Publication No. Hei 7-248365. but this publication does not teach a drive circuit including a detecting circuit.